1. Field of the Invention
The present invention relates to a scanning type display apparatus that displays an image by scanning a light beam emitted from a light source two-dimensionally.
2. Related Background Art
In some image display apparatuses for head mounted display apparatuses and digital cameras, a two-dimensional display element such as a transmission type liquid crystal, a reflection type liquid crystal and an organic EL element is used in combination with an eyepiece optical system to allow a viewer to view an image on the two-dimensional display element as a virtual image. Such image display apparatuses are required to produce high definition images.
However, in the above-mentioned two-dimensional display element, since pixels of a required pixel number are formed on the display device, pixel defects increase with an increase in the pixel number. In addition, manufacturing of the two-dimensional display element is difficult since the size of each pixel is small relative to the size of the device.
There has been also proposed an image display apparatus in which a one-dimensional type display element and scanning means are used in combination and two-dimensional display is realized by synchronizing an image displayed on the one dimensional display device and scanning to achieve two-dimensional display. However, in such apparatuses an increase in the resolution involves an increase in the number of pixels of the one-dimensional type display element, which leads to the problems same as those of the two-dimensional display element.
On the other hand, in an already disclosed image display apparatus that does not use a two-dimensional display element or a one-dimensional type display element but displays an image by scanning a light beam emitted from a light source using two-dimensional light beam scanning means. In the technology disclosed in U.S. Pat. No. 5,467,104, light beams of red, blue and green are scanned two-dimensionally in the horizontal and vertical directions, so that an image is formed on the retina of a viewer's eye through an optical system.
As per the above, in the case of the display apparatus in which a light beam is scanned, it is not necessary to form pixels corresponding to required resolution unlike with the case of a two-dimensional or one-dimensional image display element. In addition, such an apparatus will not suffer from pixel defects in principle.
In realizing such a scanning type image display apparatus, micro electron mechanical systems (MEMS) manufactured through a semiconductor process have been gathering attention for use as light beam scanning means. The light beam scanning means manufactured based on MEMS technologies are suitable for image display apparatuses, since they are small in size and light in weight and can operate at high speed. As light beam scanning means based on MEMS technologies, planar type galvano mirror and its manufacturing method (U.S. Pat. No. 5,606,447) and a light deflecting element (Japanese Patent Application Laid-Open No. H08-334723) have been disclosed. An MEMS of a two-dimensional scanning means for a head mounted display apparatus is disclosed in “Wafer Scale Packaging for a MEMS Video Scanner” SPIE, Conference #4407-29, June 2001.
The light beam scanning means based on the MEMS technologies resonates a light reflection surface by a torsion bar or the like to deflect and scan a light beam incident on the reflection surface. Since torsion is utilized, the light reflection surface is not rotated but oscillated. When use is made of such light beam scanning means, the number of scanning lines can be increased if both forward motion and backward motion in oscillation are used, namely if both scanning from right to left and scanning from left to right are utilized. Such scanning is referred to as reciprocating scanning. By effecting scanning in the direction perpendicular to the reciprocating scanning in addition to the reciprocating scanning, two-dimensional reciprocating raster scanning can be realized. In the case where an image is displayed by reciprocating raster scanning, it is important to synchronize image-drawing in the forward path and in the backward path.
Since scanning motion realized by oscillation is sinusoidal, the larger the amplitude of deflection is, the lower the speed is, and the speed becomes zero at the maximum amplitude. Therefore, it is necessary to design the scanning area in which an image is formed (or the effective scanning area) smaller than the entire scanning angle (or the entire deflection angle).
In connection with synchronization methods in scanning type image display apparatus or reciprocating scanning, a projection type display apparatus has been disclosed (U.S. Pat. No. 5,489,950). Another document discloses an light beam scanning apparatus, a oscillation member for use in a light beam scanning apparatus and an image forming apparatus equipped with a light beam scanning apparatus (Japanese Patent Application Laid-Open No. 2003-57586). Furthermore, a light beam scanning apparatus (Japanese Patent Application Laid-Open No. H09-230276), a light beam scanning apparatus and an actuator (Japanese Patent Application Laid-Open No. 2003-57577) have been disclosed.
U.S. Pat. No. 5,489,950 relates to synchronization in a display apparatus that deflects and scans a one-dimensional image using a deflecting mirror to project a two-dimensional image. This apparatus detects the angle displacement of the deflecting mirror that oscillates over a specific angle range at a constant cycle by means of a detector attached to the deflecting mirror and generates synchronization timing based on a signal of the detector.
Japanese Patent Application Laid-Open No. 2003-57586 discloses a technology for a light beam scanning type display apparatus that utilizes an MEMS micro mirror. In the technology disclosed in Japanese Patent Application Laid-Open No. 2003-57586, light beams from red, green and blue laser sources are color-composited and guided to an optical fiber optical system. The light beam exiting from the optical fiber optical system is collimated by a collimation optical system, and then made incident on horizontal scanning means to scan the light beam horizontally. The light beam from the horizontal scanning means is made incident on the vertical scanning means via a relay optical system and scanned in the vertical direction. After that, the scanned light is made incident on a viewer's eye via a second relay optical system. In this arrangement, a beam detection sensor (BD sensor) is provided between the horizontal scanning means and the vertical scanning means. An electric signal that is generated when the scanning light beam passes over the BD sensor is sent to a BD signal detection circuit. Start timing of an image signal is determined based on this signal.
Japanese Patent Application Laid-Open No. H09-230276 discloses a light beam scanning apparatus that deflects a laser beam by sinusoidal oscillation. This document discloses correction of a difference in the scanning start points between the forward path and the backward path caused by variations in the deflection frequency, and light beam emitting means is controlled in such a way that the start points coincides with each other. Japanese Patent Application Laid-Open No. 2003-57577 teaches to prevent interference of optical path by entering a second light beam used for generating a light-reception output to a deflector in addition to a first light beam for scanning an effective scanning area. The light-reception output is used for synchronizing the forward path and the backward path in oscillating operation of scanning means in a scanning type image display apparatus having the scanning means for two-dimensionally scanning a light beam emitted from a light source.
U. S. Pat. No. 5,489,950 relates to synchronization in a display apparatus that deflects and scans a one-dimensional image by a deflecting mirror to project it as a two-dimensional image. The apparatus uses a drive signal and a mirror deflection angle detection means in synchronizing the mirror and the image. Since the scanning light is not detected in this apparatus, sufficient accuracy is not realized in some cases.
In the technology disclosed in Japanese Patent Application Laid-Open No. 2003-57586, BD synchronization detection is effected based on a light beam scanned by horizontal scanning means. However, since no optical system is provided between the horizontal scanning means and a BD optical sensor, overlapping of the effective beam and the beam for beam detection (BD) is likely to occur.
Japanese Patent Application Laid-Open No. H09-230276 discloses an optical configuration in which scanning start points in the forward path and in the backward path in sinusoidal oscillation coincide with each other. However, this configuration is designed to scan a specific surface to be scanned by one-dimensional scanning means to form an image on the surface to be scanned. Moreover, in this configuration, first and second light detectors are disposed in deflection angle ranges inside the entire deflection angle range of oscillating scanning means but outside the effective deflection angle range. Accordingly, if the space between the effective deflection angle and the optical path for the light detector is narrow, interference of the optical path occurs, and a loss of the light quantity of the effective beam arises.
In the configuration disclosed in Japanese Patent Application Laid-Open No. 2003-57577, since the second beam for generating a light-reception output different than the first beam for scanning the effective scanning area is made incident on the deflector, interference of the optical path does not occur. However, use of two different beams makes the structure complex.